Within the food packaging industry it has for a long time been used packages formed from a web or a blank of packaging material comprising different layers of paper or board, liquid barriers of for example polymers and gas barriers of for example thin films of aluminium. Another type of packages that are used in the food packaging industry is packages made from polymer material and manufactured through for example blow-moulding.
To extend the shelf-life of the products being packed it is prior known to sterilize the web before the forming and filling operations, and to sterilize the partly formed packages (ready-to-fill packages, RTF packages) before the filling operation. Depending on how long shelf-life is desired and whether the distribution and storage is made in chilled or ambient temperature, different levels of sterilization can be chosen. Usually, the level “commercially sterile” is applied for packages aimed for ambient temperature.
One way of sterilizing a web is chemical sterilization using for example a bath of hydrogen peroxide. Similarly, a ready-to-fill package can be sterilized by hydrogen peroxide, preferably in gas phase.
Another way of sterilizing packaging material is to irradiate it by means of electrons emitted from an electron beam emitting device, such as for example an electron beam generator. Such sterilization of a web of packaging material is disclosed in for example the international patent publications WO 2004/110868 and WO 2004/110869. Similar irradiation of ready-to-fill packages is disclosed in the international patent publication WO 2005/002973. The above applications are hereby incorporated by reference.
An exemplary system for sterilizing packages by electron beam technology includes an electron beam sterilizing device for emitting an electron beam along a path. The device is connected to an electron beam generator that is connected to a high voltage power supply and a filament power supply. The latter transforms power from the high voltage power supply to a suitable input voltage for a filament of the generator. The filament can be housed in a vacuum chamber. In operation, electrons e− from the filament are emitted along an electron beam path in a direction towards a target. A grid around the filament is used for diffusing the electron beam into a more uniform beam, and for focusing the electron beam towards the target. Beam absorbers and magnetic fields can also be used to shape the electron beam. The electrons are exiting the sterilizing device through an electron exit window.
A system like this may as well also be used for other purposes than sterilization, for instance for curing of inks and coatings.
However, a disadvantage using irradiation is that much of the energy supplied to the system is lost when the electrons hit molecules in the surrounding environment (such as the air) and the energy is absorbed. Because of that it is difficult to make the electrons travel long distances, and therefore it is also sometimes difficult to provide a uniform electron beam dose to the object to be irradiated. It may for example sometimes be difficult to reach the portions of the object located most distant the electron beam generator.
Further, to obtain a sufficient irradiation level, the irradiation time has to be relatively long. This is inefficient from two perspectives. Firstly, the production time is affected, and in high speed production a long irradiation time is of course a disadvantage. Secondly, the longer irradiation time the more energy is used, and this considerably affects the cost.